The purpose of this research is to obtain a more accurate quantitative description of the contraction of cardiac mmuscle, by investigating the relationship between mechanical events inside individual muscle cells, and external aggregate mechanical properties of whole muscle. New and improved apparatus will be developed for measurement of muscle external force and motion, while simultaneously observing movement of intracellular structures through the microscope. By systematic analysis of intracellular movement, structural explanations will be sought for aggregate muscle properties of elasticity and contractility. Special emphasis will be given to the role of intracellular heterogeneity, and the effects of preparation and mounting artifacts. Results will be used to differentiate between and refine existing alternative contraction models for cardiac muscle, in order to understand better the normal and abnormal function of the heart.